TBH I don't like it with the standard Breeze color scheme--or any color scheme where the titlebar is a bold or highly contrasting color. The top part of the window starts to feel overwhelming IMO. I think it looks out of place with other windows where that area of color is much smaller.

Could we make the tab bar background and inactive tabs have better contrast with the active tabs without going this far?

Now it's is literally impossible to tell which tab is active when the window is inactive with the default Breeze color scheme:

This doesn't happen with all color schemes I tested but it's a problem with Breeze.

I'm uneasy approving a change that is so dependent on the user's color scheme to look good--especially if it has problems with the default color scheme. For any color scheme where the titlebar is strongly contrasting with the window background, the top of the window just looks super duper overwhelming to me. It results in a look we have anywhere else and with the default Breeze color scheme I think it looks super heavy:

The only color schemes I can find where I think it looks good are light ones like Breeze Light, Breath, and Materia Light:

It results in a look we have anywhere else and with the default Breeze color scheme I think it looks super heavy:

This looks absolutely fantastic.

All of the light options you like lack contrast to the disabled tabs. The main reason I had to stop using Breeze GTK in Chrome is that inactive tabs have the same color as the current one making it hard to see which is the current tab.

Maybe we need to take step back and come up with a solution that works for all schemes, even if it means tinting the user color for this particular control (if that's possible with CSS).